Flotation device for the treatment of water polluted by hydrocarbons

ABSTRACT

A flotation device for the separation of nonmiscible liquids, particularly the separation of water and hydrocarbon pollutants, comprising a cylindrical tank divided by partitions into a feed flow rate homogenization compartment, at least one flotation cell equipped with a cyclone-turbine and a skimming chamber with self-stabilizing chute, and a tranquilization compartment, characterized in that said cyclone-turbine is equipped with a throttling disk and vortex stabilization baffles, and that the skimming chute is equipped with lateral floats and counterweights.

BACKGROUND OF THE INVENTION

The present invention pertains to a device for the separation byflotation of nonmiscible liquids. The invention focuses morespecifically on the treatment of water polluted by hydrocarbons.

It is known to treat such mixtures of water and hydrocarbons byflotation either with dissolved air (D.A.F. system, "dissolved airflotation") or with induced air or gas (I.A.F. systems, "induced airflotation"). In I.A.F. devices, the induction of air or gas is generatedeither by means of injectors or by means of rotor-stator type turbines.The device in accordance with the present invention pertains to thislatter category. Thus, flotation devices of the I.A.F. type are knownwhich operate by means of turbine-induced injection of air or gas, whichdevices are constituted of a cylindrical tank with one or more passivezones in which the tranquilization or decantation of the oils takesplace, and with one or more active zones in which a rotor/stator unitoperates as a flotation-air injection device, and which discharge intoone or more collector chambers receiving the froth formed in the activecompartments. Information on these devices can be found in themanufacturers' bulletins and in various patents, e.g., French Patent No.2,605,898.

The role of the turbines is to induce, via rotation of the rotor, adepression in the stack above them, thereby drawing in the covering gasand directing it against the stator, thereby creating around the statorthe cloud of gas which will generate the flotation. These turbines aresimply constituted, in accordance with the prior art, of a rotor withmobile blades inside the cage of a stator with fingers of rectangularsection. These units have a certain number of drawbacks:

a) instability of the flow rate of the air or gas due to the rupture ofthe diphasic flow profile in the rotor's stack; the rotor rotates in adiphasic water/air medium, which results in a considerable fluctuationin the electrical power absorbed;

b) periodic ascension of the vortex up to the air-induction point,leading to water being directed onto the froths, thereby inducing theirpartial redissolution;

c) the flotation effect is not assured with turbines handling flow ratesgreater than 50 m³ /hour since the turbines only act as agitatorsbecause the depression created by the rotor is not sufficient to carrythe covering gas to the stator; and

d) impossibility of varying the ratio between the water flow rate andthe air or gas flow rate.

For these reasons, even the operation of these units at the pilot scaleis acceptable, when the equipment described in the prior art is producedat industrial scale the flotation effect is not satisfactory.

The skimming chutes in accordance with the prior art areself-stabilizing. Nevertheless, they are not equipped with anypossibilities for regulating their immersion, their skimming perimeteris very small, their horizontality is very difficult to maintain andtheir flotation is uncertain, i.e., the chute can partially emerge fromthe water or leak; the result is that the skimming of the froths is veryimperfect and the device becomes totally unstable after several weeks ofoperation.

SUMMARY OF THE PRESENT INVENTION

The device in accordance with the makes it possible to resolve thesedrawbacks. It is constituted (FIG. 1) of the cylindrical tank divided bypartitions into several distinct compartment: a compartment forhomogenizing the feed flow rate (1), flotation cells (2) and atranquilization compartment (3), possibly followed by a pot acting asthe hydraulic seal with the exterior.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of the flotation device in accord with thepresent invention;

FIG. 2 is a cross-sectional view of a cyclone-turbine(4) of FIG. 1;

FIG. 3 a partial plan view of throttling disk (30) of FIG. 2;

FIG. 4 is a sectional view of a pot (21) in accord with the presentinvention;

FIG. 5 is a top plan view of the pot of FIG. 4;

FIG. 6 is a sectional view of chute (5) in accord with the presentinvention;

FIG. 7 is a top plan view of the chute of FIG. 6; and

FIG. 8 is a partial section view of a portion of the last compartment inthe device depicted in FIG. 1.

DETAILED DESCRIPTION

Referring to FIG. 1 each cell is equipped with a cyclone-turbine (4) anda self-stabilizing chute (5) in common with an adjustable cell. Thechute can be displaced vertically by means of a skimming chamber (6),the fixed walls of which (47 in FIG. 6) act as a guide-support for thechute.

The cyclone-turbines (FIGS. 2 and 4) are of the stator-rotor type. Theyare each equipped with a throttling disk (30) making it possible tocreate a depression which is sufficient to overcome the water columnabove the rotor. This disk (FIGS. 2 and 3) is several millimeters thickand is installed between the flanges of the stack and the cage of thestator (35). It is made of stainless steel and has a diameterapproximately equal to the exterior diameter of the rotor. Baffles (31)are welded on the inside walls of the rotor's stack (32). There arepreferably three baffles which are arranged symmetrically about thirtycentimeters from the flange (33-34). The baffles are preferably made ofsteel. Indicatively, their dimension are between 100×40×6 mm and200×60×4 mm. These baffles make it possible to obtain a flow profilewith a stable vortex. The combination of a tangential velocity of therotor with the throttling disk and the vortex-stabilization baffles makeit possible for the turbine to generate a regular diphasic gas/waterflow. The water: gas flow rate ratio is on the order of three. Thepurpose of a stack crosspiece is to prevent ascension of the vortexwhich would result in water being directed onto the froths formed on theexterior of the stack. It should be noted that the use of these improvedturbine-cyclones is not limited to flotation devices but can be extendedto any gas-liquid application.

The stator (35) of the cyclone-turbine in accordance with the inventionis advantageously made of polypropylene; the rotor, which is made ofcommon material (stainless steel) has a reduced number, preferably six,of blades. In addition, these cyclone-turbines are installed in a mannersuch that it is possible to put them in either a high or low position,with the high position being the ordinary position and the low positionbeing obtained by inserting a piece between the coupling flanges (37).The thickness of this piece makes it possible to act directly on the gasflow rate.

The improved chutes (FIGS. 6 and 7) in accordance with the inventionhave two vertical floats (41), an exterior skirt (42), two lateralfloats (43) for which the skirt acts as a guiding mechanism, and twocounterweight units (44). Skimming of the froths takes place in ahomogeneous manner over the wide perimeter of the chute (45) and therecovered froths are discharged (A) via the bottom of the chambers.

The dimensions of the chutes take the following factors into account:

(i) overall dimensions of the flotation device,

(ii) froth flow rate,

(iii) range of variation in the flow rate of the water to be treated,

(iv) dimensions of the support flanges of the cyclone turbines (7), and

(v) volume of the vertical floats (41), lateral floats (43) and of thevolume of air or gas retained in the space between the exterior skirt(42) and the floats and which constitutes the pneumatic seal (46).

The dimensions of the floats are in accordance with the followingprinciples:

a) the sum of the volumes of the vertical and lateral floats multipliedby the density of the liquid to be treated must be greater than thetotal weight of the chute; and

b) the sum of the mean volumes of the vertical and lateral floats of thepneumatic seal multiplied by the density of the liquid to De treatedmust be equal to the total weight of the chute when it is in the highposition.

For the engineer, these principles will be translated into equationswith multiple unknowns. These equations can only be solved by theselection of a certain number of dimensions, such as the dimensions ofthe internal floats.

The real weight of the bare chute must remain slightly lighter than thetheoretical weight; equilibrium will be achieved by addingcounterweights. The use of this counterweight solutions has thefollowing advantages:

(i) for the designer of the chute, it facilitates the numerical solutionof these two equations by carrying out first order simplifications;

(ii) flexibility at the operating level, since in this strongly aeratedmedium its apparent density is different from that of the water to betreated, especially at the flow surface; and

(ii) easy correction of the control of the immersion and horizontalityof the chute, the weight of which is modified during operation becauseof encrustation by bituminous or other products, by acting on thecounterweights without disturbing the overall operation of the flotationdevice.

The chutes are made of stainless steel or any other material capable ofresisting the aggressive nature of the medium (high salinity, presenceof hydrocarbons, H₂ S, CO₂, etc.). If plastic is used for theirconstruction, they must be made heavier by ballasting with lead beads.These chutes operate practically without requiring any control orintervention. This advantage is reflected in the general operation ofthe flotation device, which requires almost no maintenance.

the last compartment of the tank (3) assures tranquilization of theeffluent and separation out of the oil droplets larger than circa 60micrometers which escaped the last flotation cell. Tranquilization isproduced in the compartment (see FIG. 8) by means of hydraulic retentionmaintained by the positioning of the outlet pipe (72). The decanted oilsare recovered by a chute with an adjustable sill (71); siphoning out ofthe water is made impossible by an aeration tube (73). Skimming of theoils can be carried out either continuously or intermittently.

In the case of an installation operating under gas cover,tranquilization is completed by a pot (21) installed downstream of theflotation device (FIGS. 4 and 5) and which assures tightness. The waterlevel in the compartment (3) to which it is connected is regulated bymeans of an adjustable sill (22) accessible via the top part (23) of theequipment. The body of the pot is constituted of a tube with a diameterapproximately twice the diameter of the outlet pipe (24). The sill (22)is connected to the outlet pipe by a helicoidal thread, and itselevation is easily achieved by turning it in one direction or theother. The height of the water in the pot and the elevation of the potare determined in a manner such as to overcome the pressure of thecovering gas (which is on the order of several tens of millibars). Thissolution is particularly advantageous because it simplifies considerablythe architecture of the flotation system's cylindrical tank, whichresults in improvement of the device, the last compartment of which isessentially devoted to skimming the oils which are decanted there, aswell in appreciable cost savings in construction. These are additionalcost savings because there is no need for delicate instruments such as alevel detector and a regulation blade which were required in the priorart for installation of the hydraulic seal.

Created in this manner, these induced air injection devices are improveddevices which make it possible to correctly control the air and gas flowrates and the water: air (gas) ratio, and to control the production anddischarge of the froths. The material used for these devices are light,their construction costs are low and their consumption of energy isreasonable.

While the invention has been described in connection with a preferredembodiment, it is not intended to limit the scope of the invention tothe particular form set forth, but on the contrary, it is intended tocover such alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A flotation device for the separation of oils,creams, or froths floating on the surface of aqueous liquids, comprisinga cylindrical tank divided by partitions into a feed-flow ratehomogenization compartment, at least one flotation cell equipped with(a) a cyclone-turbine comprising a rotor with mobile blades housed in astator cage and a stack positioned above the cyclone-turbine and (b) askimming chamber with a self-stabilizing chute, and a tranquilizationcompartment, said cyclone-turbine having a throttling disk positionedbetween said stator cage and said stack and vortex stabilization bafflesattached to the inside wall of said stack, said skimming chute beingdisplaced vertically with respect to the surface of the aqueous liquidin said at least one flotation cell, and said skimming chute beingself-stabilizing and comprising an exterior skirt having attachedthereto two counterweights, two lateral floats, and two vertical floatswhich act to maintain said skimming chute at the surface of the aqueousliquids.
 2. The flotation device of claim 1 also including a connectingpot attached to said flotation device at said tranquilizationcompartment to assure a seal between a capacity under gas cover and theexterior, said pot comprising a cylindrical body inside of which extendsvertically an outlet pipe, having an adjustable sill displacedvertically in relation to the interior upper end.